I keep a roll of masking tape in my kitchen drawer to identify what and when I put leftovers in the frig. When I have left overs I write on the tape what it is and the date and tape it on the container. I do this with boiled eggs also. This way I can realize how long something has been in the frig and know what it is without always having to open it . By marking my leftovers this way I've wasted less food.